share/examples/oci: Run 'pkg update' with IGNORE_OSVERSION set

This allows the image build to work even if the host's OSVERSION doesn't
match the pkgbase OSVERSION which happens if the host's patch level or
release version isn't the same as pkgbase.

Also fix the image name so that Podman/Buildah will fetch it
automatically, removing the need for users to pull an image and retag it
before using this example.

Reviewed by:		dch
MFC after:		3 days
Differential Revision:	https://reviews.freebsd.org/D50596

share/examples/oci/Containerfile.pkg

@@ -6,7 +6,7 @@
 ARG version=14.snap
 
 # Select freebsd-runtime as our starting point.
-FROM localhost/freebsd-runtime:${version}
+FROM ghcr.io/freebsd/freebsd-runtime:${version}
 
 # A list of package(s) to install
 ARG packages
@@ -15,7 +15,10 @@ ARG packages
 # use for downloading pkg since the freebsd-runtime image has both FreeBSD and
 # FreeBSD-base pkg repo configs installed and FreeBSD-base does not contain the
 # pkg package.
-RUN env ASSUME_ALWAYS_YES=yes pkg bootstrap -r FreeBSD && pkg update
+#
+# Set IGNORE_OSVERSION to allow building e.g. FreeBSD-14 images on
+# FreeBSD-15 hosts.
+RUN pkg bootstrap -y -r FreeBSD && pkg -o IGNORE_OSVERSION=yes update -f
 
 # Install some package(s).
 RUN pkg install -y ${packages}